Fixed `LayerMapping` to work with PostGIS geography fields; removed `…

…LayerMapping.geometry_column` and replaced with `LayerMapping.geometry_field` because getting the `geometry_columns` entry was completely unnecessary, and only the geometry field instance is needed; cleaned up and fleshed out the `geogapp` tests.
git-svn-id: http://code.djangoproject.com/svn/django/trunk@11983 bcc190cf-cafb-0310-a4f2-bffc1f526a37